Oligoclase
| Oligoclase | ||
|---|---|---|
Specific gravity 2.64 to 2.66 | | |
| Refractive index | nα = 1.533–1.543; nβ = 1.537–1.548; nγ = 1.542–1.552 | |
| Birefringence | 1st order | |
Oligoclase is a rock-forming
The albite:anorthite molar ratio of oligoclase ranges from 90:10 to 70:30.Oligoclase is a high sodium feldspar crystallizing in the
specific gravity is 2.64 to 2.66. The refractive indices are: nα = 1.533–1.543, nβ = 1.537–1.548, and nγ = 1.542–1.552. In color it is usually white, with shades of grey, green, or red.[1]
Oligoclase is a common mineral in the more
silica-rich varieties of igneous rock and in many metamorphic rocks
.
Name and discovery
The name oligoclase was given by
J. J. Berzelius in 1824, and was named by him soda-spodumene (Natron-spodumen), because of its resemblance in appearance to spodumene.[1]
Occurrence
Perfectly colorless and transparent glassy material found at Bakersville, North Carolina has occasionally been faceted as a gemstone. Another variety more frequently used as a gemstone is the aventurine-feldspar or sun-stone found as reddish cleavage masses in gneiss at Tvedestrand in southern Norway; this presents a brilliant red to golden metallic glitter, due to the presence of numerous small scales of hematite oriented within the feldspar structure.[1]
Oligoclase occurs, often accompanying
metamorphic rocks formed under transitional greenschist to amphibolite
facies conditions.
Schiller iridescence
Some examples are called
exsolution lamellae on cooling in the peristerite miscibility gap
, ~An5-An18.
References
- ^ a b c d e f One or more of the preceding sentences incorporates text from a publication now in the public domain: Spencer, Leonard James (1911). "Oligoclase". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 20 (11th ed.). Cambridge University Press. p. 82.
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
- Webmineral data
- Mineral galleries
- Mindat